Removable shearing machines have been used as attachments on the end of the arm or boom of power shovels to shear steel sheets, steel reinforcing bars and the like. Various types of upper jaw shapes, lower jaw shapes and blade shapes have been developed according to the nature of the work. In the most generally used shearing machines, an upper blade and lower blade are disposed on an upper jaw and lower jaw, respectively. The blades are opened and closed crossing each other in a scissor fashion. In another type of shearing machine, a bend at the central portion of either one or both of the upper and lower blades prevents the steel to be sheared from slipping out past the blade tips once it is caught between the upper and lower blades.
Prior art shearing machines have the disadvantages described below.
In shearing machines having linear upper and lower blades that can open and close and that are pivoted around an axis:
(1) The space between the upper and lower blades is wide toward the outer tips of the blades, while a small triangle is formed at the base. The workpiece is mainly sheared toward the outer tips, away from the pivot. Thus, the output of the cylinder driving the upper and lower jaws operates inefficiently on the workpiece.
(2) In a pick-up operation, a workpiece is scissored between the tips of the upper blade and lower blade. Because of the left-right offset between the tips of the upper and lower blades, the workpiece rotates during the scissoring, thus preventing stable pick-up.
(3) Because the upper and lower blades are linear, the workpiece can slip toward the tip during shearing.
In shearing machines having bent blades, the problem described in (3) is overcome. However, regarding the problem described in (1), the output from the cylinder is used more efficiently than shearing with the tips of the blades, since the shearing of the workpiece takes place mainly toward the center of the upper and lower blades, rather than toward the tips. However, efficient shearing at the base portion of the blades can not take place. Also, the problem described in (2) above remains unresolved.
Another problem with prior-art shearing devices is:
(4) Bent blades result in high force concentration locally at the bend during shearing. This leads to greater wear at the bend compared to other portions of the blades, thus shortening the lifespan of the blades.